Radiographic imaging involves the measurement of the attenuation of x-ray radiation by a subject and the weighting or processing of such measurements to provide a two-dimensional image of the subject. The process has been known since approximately the time of discovery of x-rays in 1895. The subject is usually a portion of a human or animal, such as a chest, teeth, abdomen, or breast, or a portion of a structure or object, but may be an entire human, animal, structure or object. In addition, a substance may be introduced into the subject, such as a contrast agent, to improve contrast in the resulting image. Radiographic imaging is generally used for characterization of the subject, such as detection of the presence or absence, estimation of the size, or estimation of any other geometrical or physical property of any part of the subject, including but not limited to tumors, calcifications, organs, portions of organs, features, boundaries, foreign substances or anomalies, for diagnostic, medical, dental, testing and other purposes of a user. The term user is employed to denote any individual who is characterizing the subject, such as medical or dental professionals or paraprofessionals, engineers, and operators of equipment under their guidance.
In its simplest form, radiographic imaging is conducted by providing a source of x-ray radiation, the subject and a detector which serves to detect and record the radiation transmitted through the subject during the total exposure time to the x-ray radiation. The most common and simplest form of detector is x-ray sensitive photographic film. In a conventional radiographic imaging system, radiation is transmitted through the subject and then is detected by a substantially two-dimensional plane of x-ray sensitive film. Frequently a film-screen combination replaces the x-ray film. In digital radiographic imaging or digital radiography other detectors are employed, such as an image intensifier combined with a television-scan or charge-coupled device (CCD) camera, or a combination of a screen and CCD and others.
Commonly-available x-ray detector configurations include point scan, slit scan, and slot scan systems and fixed two-dimensional image receptors, optionally with fiber optic or lens adjuncts. These adjuncts direct visible light photons to the detector when a screen is employed to transform x-ray photons to visible light. Scanning devices are used because the division of two-dimensional image receptors and corresponding x-ray illuminators into a plurality of sub-arrays allows the use of narrow x-ray beams, which in turn reduces undesirable effects of scattered radiation, and because a sub-array requires a smaller detector than a complete array.
In commonly-used medical radiographic imaging devices, including digital radiography systems, the quantity measured in the x-ray detectors for each picture element (pixel) of the two-dimensional image (and used for display or further processing) is the total energy absorbed by the detector over the total exposure time, with some efficiency less than 100 percent. This total energy in each pixel is related to the response of the detector to all x-ray photons of different energy levels bombarding the pixel at different times. Additionally, some detectors, particularly film, have a non-linear response to the energy level of the x-ray photons detected. The effect is that the total energy measured includes a non-linear weighting factor in which high energy x-ray photons are weighted more than lower energy x-ray photons.
That an x-ray image is based on the sum of the responses of all x-ray photons transmitted through the subject is a problem because some of the x-ray photons emitted by the x-ray source have a high energy which are almost never attenuated by the subject. These x-ray photons thus contribute essentially little or no useful information. Other x-ray photons have an energy level such that they are only seldom attenuated, and thus provide little information. In contrast, the passage of a lower energy photon through the subject is less likely, but often provides more information. By including the response due to x-ray photons which provide little information, the quality of the resulting image is reduced.